Ultrasonic micrometer

ABSTRACT

The invention generally relates to the method of measurement of distances through reflexion of ultrasonic pulses on a reflecting surface and determination of the time interval of propogation of the resulting echoes from the reflecting surface to the transducer. The instant invention more particularly discloses a micro method based on said method and including means for focusing the ultrasonic pulsed ways at an accurately predetermined point on the reflecting surface.

ilnited Mates tent Dory n51 ann? 1451 Mar.28,1972

[54] ULTRASONIC MICROMETER [72] Inventor: Jacques Dory, Meaux, France[73] Assignee: Corporation Realisations Ultrasoniques" [22] Filed: July9, 11968 21 Appl. No.: 743,415

Related US. Application Data [63] Continuation-in-part of Ser. No.495,463, Oct. 13, 1965, Pat. No. 3,431,774, Continuation-impart of Ser.No. 508,348, Nov. 17, 1965, Pat. No.,,3,423,992, Continuation-impart ofSer. No. 678,929, Oct. 30, 1967, Pat. No. 3,454,922.

[30] Foreign Application Priority Data July 10,1967 France ..1 13738[52] 111.5. C1. ..73/67.8 R, 73/715 [51] lint. Cl. ..G0ln 29/00 [58]Field of Search ..73/67.5, 67.6, 67.7, 67.8, 73/679, 71.4

[56] References Cited UNITED STATES PATENTS 2,985,018 5/1961 Williams..73/67.5 X 3,209,591 10/1965 Lester 73/675 X 3,287,963 11/1966 Stanyaet a1. ..73/67 9 3,379,051 4/1968 Zeutschel et al. 73/675 X 3,427,8662/1969 Weighart ..73/67.7 2,995,926 8/1961 Dory ...73/67.8 3,394,5857/1968 Davey ..73/67.7

Y., 1962, pp. 62. Smith; R. T., Stress Induced Anisotrophy in Solids,Ul-

trasonics, July-Septfl963, Vol; 1''- 2, 1963- 1964, pp. 138- 139.Goldman; R.. Ultrasonic Technology, Reinhold Publishing Co., N. Y.,1962, pp. 71- 78.

Primary Examiner--Richard C 1 Oueisser Assistant Examiner-Arthur E.Korkosz Att0rneyWilliam Anthony Drucker [5 7] ABSTRACT The inventiongenerally relates to the method of measurement of distances throughreflexion of ultrasonic pulses on a reflecting surface and determinationof the time interval of propogation of the resulting echoes from thereflecting surface to the transducer.

The instant invention more particularly discloses a micro method basedon said method and including means for focusing the ultrasonic pulsedways at an accurately predetermined point on the reflecting surface,

3 Claims, 12 Drawing Figures PATENTEUMAR28 :972 3,651,687

SHEET 1 OF 3 PMEWWMARN 1912 SHEET 3 []F 3 ULTRASONIC MICROMETERREFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of copending applications,

Ser. No. 495,463 filed Oct. 13, 1965, now U.S. Pat. No.

Ser. No. 508,348 filed Nov. 17, 1965, now US. Pat. No.

Ser. No. 678,929 filed Oct. 30, 1967, now US. Pat. No.

Dial indicators or micrometers used conventionally for the measurementand checking of the dimensional characteristics of mechanical partsemploy Bordas method: the indicator is set to zero when the standard isinserted into the balanced measuring line, and it is then replaced, insaid line, by the part i to be measured, so that the indicator gives thedeviation value between the part and the standard.

These devices are mechanical and most often comprise a fixed anvilsupporting the parts and a test probe which is contacted with the partin order to carry out the measurement.

The precision of these devices can be as much as one onehundreth mm.provided the reaction of the test probe on the part is not too great(otherwise the difference in deflection due to the diiference in weightbetween the standard and the part leads to a source of error) and thatthe measurement stress exerted on the test probe does not undergo undulyhigh fluctuations. When a moving or rapidly vibrating part is to bechecked, they become difficult to use because of their inertia.Generally, numerous problems arise in use: as an example, the problem ofequating the temperatures of the standard and the part to be measuredand the problem of return, in the case of bore measurements.

Present day techniques for dimensional measurements further employpneumatic or electrical systems.

However, pneumatic systems have a relatively large inertia and a reducedmeasurement range while electrical devices (such as capacitive or mutualinduction gauges) are linear on a short distance range only, theirindications, furthermore, depending on the nature of the walls, which,in certain cases, must be plated.

As far as vibrating system measurements are concerned accelerationmeters are used in addition to the previously mentioned devices. Theseinstruments have the disadvantage of requiring an intimate contact withthe vibrating component whose operation may thus be disturbed.

In general, the fields of application of each of the devices referred toabove are relatively limited, which makes it necessary to have severalclasses of instruments available if various measurement problems are tobe solved.

Furthermore, the present day trend is to use, in measuring lines,detectors producting anamorphosis, i.e., the conversion of the quantityto be measured into an electrical quantity, the latter beingadvantageously in numerical form (electrical pulses).

It may be concluded, finally, that known devices for dimensionalmeasurements are not provided with all the conveniences which should beexpected in a modern instrument.

Accordingly, it is an object of the present invention to provide such aninstrument through the application of the known principle wherebydistances are measured by determining the go and return propagation timeof pulses in a high frequency directional ultrasonic beam reflected by asurface of the medium to be controlled, so as to measure with a highprecision, the position or displacement of a surface or a point in asurface, in a dynamic or static state.

Known distance measuring devices applying said principle are in no wayadapted for use as a micrometer, which requires the measurement of adistance to be carried out without any mechanical contact with the part,at a distance, generally less than 10 cm., which can reach a lower limitof the order of 1 mm., and with a precision higher than 10' as arelative value, and ranging from one tenth mm. to 1 micron as anabsolute value.

Indeed, the detector or probe unit of these devices comprises anelectro-acoustical transducer generally designed and constructed so asto measure relatively large distances, and therefore, so as to transmitultrasonic pulses at a relatively low rate.

Certain devices transmit ultrasonic pulses at a sufficiently high rateto allow measurements of a relatively small distance, but in that case,the transducer is in contact with the surface of the medium (solid orliquid, for example) of the mechanical part whose thickness, forexample, is to be measured, which obviously facilitates the solution ofthe ultrasonic propagation problem between the transducer and the mediumstudied, but is not suitable for the construction of a micrometer.

It should be mentioned, in addition, that the known devices referred toabove, do not make it possible to localize, with precision, theultrasonic impact zone on the surface to be controlled. 7

It is another object of the invention to provide a distance measuringdevice through reflection of ultrasonic pulses, especially designed andconstructed to be used as a micrometer, and, to this effect, capable offocusing an ultrasonic beam at a well-determined point of the surface tobe controlled and of carrying out a precise measurement of a smalldistance in air.

According to a preferred embodiment of the invention, the probe unitcomprising such a device essentially consists of an active blade made ofa piezo-electric material, inserted between a block of a substancehaving a high damping coefficient with respect to ultrasonic pulsesgenerated by said blade and of a blade, consisting of a substance havinglow absorption characteristics, whose thickness is, preferably, equal toa quarter of the wave length and of a spherical surface which comprisesthe transducer emission surface, and possesses a curvature such that theultrasonic beam emitted at a distance of several millimeters may befocused.

The damping block makes it possible for the transducer thus constitutedto generate pulse trains of very brief duration (for example, severalmicro-seconds), and whose recurrence frequency can, as a result, be veryhigh, while the matching of acoustical impedances obtained by the bladeat a quarter of a wave makes it possible to collect echoes of sufficientamplitude to make them useful for the receiver comprising the device, inspite of the fact that the propagation takes place in air, and, due tothe damping necessary to obtain pulse trains of very brief duration, byusing a very low acoustical energy.

Ultrasonic beam focusing makes it possible to localize precisely a pointon the surface of the part where the control is being effected.According to a preferred embodiment of the invention, two light sources,arranged symmetrically with respect to the transducer and associatedwith an appropriate optical system, direct two beams towards said pointwhich converge on the ultrasonic beam focusing point.

Other particularities, as well as various advantages of the invention,will appear more clearly as a result of the following description.

In the accompanying drawings:

FIG. 1 is a schematic diagram of an ultrasonic micrometer capable ofeffecting static as well as dynamic measurements.

FIG. 2 is a diagram of the probe unit comprising such a micrometer.

FIG. 3 is a section of the active part of the transducer.

FIG. 4 is a phase-shifting device used in the apparatus of FIG. 1.

FIGS. 5 and 6 illustrate the operation of the device of FIG. 1 whenapplied to the study of a vibration.

FIG. 7 is a diagram of a device to be used in recording level curves.

FIG. 8 is a schematic representation of an acoustical micrometer forcarrying out differential measurements.

FIG. 9 shows wave forms at different points of the device in FIG. 8.

FIG. 10 illustrates the use of this apparatus for the measurement of aflow rate.

FIG. 11 is a diagram of an acoustical micrometer to be used in carryingout a precise measurement of thicknesses, and,

FIG. 12 shows wave forms at different points of the apparatus of FIG. 1l.

The device shown in FIG. 1 comprises an electric pulse generator 1 of ageneral type known per se, actuating an electro-acoustical transducer 2.Generator 1 comprises a high frequency oscillator (advantageouslyranging from 1 to 3 MHz) and means for chopping the high frequencyoscillation into pulses of brief duration (advantageously of the orderof several micro-seconds).

As will be explained later, the emission times of these pulses can besynchronized, with a lag which may be adjusted with respect to time,with the vibration of part P to be controlled, when a study of thevibrating part is to be made. This result is obtained by means of afixed detector 3 which may comprise a single microphone placed in theimmediate vicinity of the vibrating surface, an electrostatic detector,or any other appropriate means.

The electric oscillation generated by detector 3 is amplified by anamplifier 4 and phase-shifted by a phase-shifter 5, in which a countercauses the phase-shifting to vary before synchronizing, in a mannerknown per se, the transmission of impulses by generator 1.

Transducer 2' converts these electric pulses into acoustical pulseswhich are reflected from surface P. The echoes are converted by thetransducer into amplified electric signals and are detected by areceiver 6 of a known type.

A measuring device 7, known per se, is connected to the output of thereceiver and may be either of the analog or numerical type.

In the first case, it delivers a continuous voltage which isproportional to the time elapsed between the emitted pulse and the echo,and in the second case comprises an electronic counter which measuresthis time by counting pulses having a fixed rhythm provided by a timebase.

In both cases, a measurement of distance d (FIG. 1) is obtained, theresult of the measurement being either recorded or displayed in acontinuous manner, which makes it possible to obtain a continuousindication of dimensions and to record tolerances.

Devices 1, 6 and 7 are advantageously provided with improvements whichare the subject of French Patent applications filed by the applicant onNov. 25, 1964, for: Device for measuring thickness and distance by meansof ultrasonic pulses" (hereafter referred to as: first application) onFeb. 8, 1966, for: Process and device for digital measurement ofdistances by means of ultrasonic pulses (hereafter referred to as:second application") on Feb. 10, 1966, for: Process and device foranalog measurement of distances by means of ultrasonic pulses (hereafterreferred to as: third application") and on Nov. 8, 1966, for: Apparatusfor measuring levels by reflection of ultrasonic pulses (hereafterreferred to as: fourth application).

The device described in the first application is designed to eliminateerrors in measurement due to the disappearance of the echo, as a result,for example, of a turbulence in the air space d crossed by theultrasonic beam.

It can generally be replaced, within the scope of this invention, by asimpler device of the anti-fading type.

The devices which are the subject of the second and third applicationsare designed to eliminate errors in measurement due to a change inpropagation characteristics, for example, due to a variation intemperature. It is fundamental, in an acoustical micrometer, to providefor such a device, or an equivalent device due to the precision ofmeasurement desired.

The device described in the fourth application is designed to correcterrors due to the imperfect form of the echoes. It is generallypreferable to provide such a device in an acoustical micrometer eventhough, because distance d is small, this source of error is relativelyinsignificant.

Transducer 2 is preferably of the type shown in FIGS. 2 and 3.

As shown by the longitudinal section in FIG. 3, in which the relativethickness of layers 21 and 22 has been exaggerated for purposes ofclarity, the active component comprises a blade 21, advantageously madeof a piezo-electric ceramic material, covered by a blade 22 made, forexample, of plastic material with an appropriate thickness to enable itto act as a quarter wave blade. These two blades are externally concave,so as to focus the ultrasonic beam emitted at an appropriate distance d.The transducer should of course be changed when modifying this distance,depending on the required application.

Blades 21 and 22 are glued to a suitable loaded block made of plasticmaterial 20 which acts as a damper of acoustical vibrations: thanks tothe presence of this block, the transducer converts a brief electricalpulse into a single acoustical pulse of the same profile. It isimportant, indeed, in the application of the micrometer, that thegenerated acoustical pulse be very brief and not be followed by a trainof interfering oscillations which might hinder the reception of echoesthe latter having a very small amplitude and being received by atransducer a vary short time after the emitted pulse provoking them.

Blade 22 improves acoustical coupling between the acoustical vibrationgenerating component 21 and the air in which the ultrasonic pulses areto propagate. Insertion loss due to the difference in acousticalimpedance existing between the air and the active component is thusreduced. In practice, this reduction of insertion loss is indispensablein the construction of an acoustical micrometer. Indeed, because thetransducer must be highly damped, for reasons that have just beenstated, a large part of the acoustical energy generated by the activecomponent is dissipated in block 20. But the relatively weak acousticalenergy available rhust give rise to an available echo in spite of itspropagation in air, i.e., in a medium with unfavorable propagationcharacteristics.

It must be well understood that acoustical matching and dampingproblems, which are particularly difficult in the construction of amicrometer, may have other solutions than the one described, forexample, the use of a highly coherent acoustical source, comprisingseveral layers of materials having very low acoustical impedances, or aprogressively variable impedance.

The solution described has the advantage of being both simple andefiicient.

The curvature of the emitting surface (external surface of blade 22) isdetermined so as to produce a focused beam having a section less than Imm. from the level of the focal spot 23 (FIG. 2).

An optical device, drawn as two light sources comprising lamps 24-25respectively contained in opaque tubes 26-27 provided with openings28-29 and associated with respective condensers 30-31, generates twolight beams whose focal spots coincide with focal spot 23. This devicemakes it possible to set the detector at the correct distance. Indeed,as long as surface P, placed at P, for example, cuts the ultrasonic beamoutside its focal point, two light spots T T can be observed which areset ofi" with respect to one another. The detector must therefore bedisplaced in a direction perpendicular to surface P until these twolight spots coincide.

It is evident that this precise setting of the sounding point on thefocal spot of a well focused ultrasonic beam is particularly importantwhen, in a study of a vibrating system, the localization of very closelyspaced vibration nodes and antinodes is to be detected.

Such a setting is, however, valuable even when effecting a purely staticcontrol: it improves the localization of the point whose value wasmeasured and, in addition, makes it possible to reflect a largerfraction of the acoustical energy emitted towards the detector.

The displacement of the detector so as to set it into position and scansurface P is carried out, either by hand, or by means of an automaticdevice 8 (FIG. 1). A filter 9 has been inserted between amplifier 6 andmeasuring device 7 so as to eliminate the various unnecessary componentsof the signal received, as will be explained later FIG. 4 shows aparticularly advantageous embodiment of phase-shifter 5.

A transistor 32, polarized by a source of continuous voltage throughresistors 33-34-35, receives on its face, the output signal of amplifier4. A variable capacity diode 36 and a resistor 37 connected in seriesare connected in parallel on resistors 33-34l connected in series. Thecommon point to diode 36 and resistor 37 is connected to device 1.Counter applies a voltage to the terminals of diode 36, through atransformer 38.

Counter Ml has an analog output, i.e., the voltage which it generatesincreases in a stairway manner by equal amplitudes up to the levelcorresponding to the maximum capacity of the counter and is thenreturned to zero, after which it begins to increase again, each step inthe stairway corresponding to a period of the vibration detected bydetector 3.

7 However, the phase-shift provoked by the device of FIG. 4 in saidvibration is proportional to the voltage applied to the terminals of thevariable capacitor diode. This phase-shift therefore, increases by awell determined quantity from one period to the next.

This results finally in the impulse emitted by generator i beingphase-shifted in a regularly increasing manner with respect to vibrationV, as shown by the wave forms in FIG. 5, which shows the positions ofseveral emitted successive impulses E, E etc. Phase shifting has beenexaggerated, to make it clearer. in reality, the phase shifter isadjusted so that the pulse will scan a complete period of the vibration,for example, in 50 emissions.

The time interval separating the emission of each pulse from thereception of the corresponding echo is proportional to distance d at theinstant of emission, the variation of this distance being equal to theamplitude of the vibration at the instant of emission. This variationhas, of course, in the example under consideration, a period equal to 50times the period of vibration.

Filter 9 will therefore have to be arranged so as to cut out thisrelatively low stroboscopic modulation frequency. It will also have tocut out the emission frequency of pulses E,; E etc; the continuouscomponent of the signal shown in FIG. 5 will also be cut out.

It should be observed that the rate of analysis of a vibration, usingthis procedure, is relatively slow.

In a varying embodiment of the device, it is possible to have theamplifier 4i (FIG. ll) function as a frequency doubler, and to regulatethe phase-shift provided by device 5 by means of a manuel controi, so asto obtain the coincidence of pulses E,, E etc. with the peaks ofvibration V (FIG. 6) this coincidence being detected by the fact thatthe output signal of amplifier 6 will then have its maximum amplitude.

In this variation, device 7 will have to be arranged so as to measurethe peak amplitude of the vibration The output voltage of amplifier 5is, in this variation, much easier to filter, and the rate of analysisis more rapid. In the case where the vibration frequency is not higherthan a few dozen hertz, the transmitter synchronization device,consisting of phase-shifter 5 and components 3-4 -MI of FIG. 1, can bedisconnected, device 7 then supplying several values (approximately 50,for example) of the vibration amplitude for each of its periods. Indeed,it is not necessary, in this case, to use a stroboscopic method, forthere is then no difficulty in using a pulse recurrence frequency, forexample, 50 times higher than the vibration frequency such a recurrencefrequency can have, for example, for a 50 Hz. vibration frequency, avalue of 2.5 kHz., i.e., a time of one twenty-five hundredths secondwill be available, which is a very sufficient transmitting time, toallow damping of interfering echoes between two successive pulses.

On the other hand, in the case where the vibration frequen cy to bestudied reaches, for example, several kHz, the application of thissimple method is practically impossible, due to the fact that themaximum pulse frequency that may be used is limited by the fact that itis necessary to wait, in order to transmit a new pulse, that theinterfering echoes originating from the preceding pulse be damped. Thecomplete device of FIG. I must then be used, in one of the two waysdescribed above.

7, shows one embodiment of measuring device 7 of FIG. I, in the casewhere the latter is to be used for recording level curves defining thestationary wave system of a vibrating part. Amplifier 6 in FIG. Iactuates a vibration peak amplitude detector 71, which is connected to arecorder 72, on the one hand, directly (conductor 73), and on the otherhand, through threshold trigger circuits 74a, 74b, 74m followed byshunting circuits 75a, 75b.... 75n.

Each of the trigger circuits 74a, "Mb... 7411 transmits a calibratedgating pulse to the corresponding shunting circuit whenever thevibration peak amplitude reaches the threshold for which it was set, therespective trigger circuit thresholds being increasingly larger so as todefine different levels. The

shunting circuit thus provides a pulse to recorder 72, which is arrangedso as to mark a point at each pulse. The writing member of this recordermoves in synchronism with the scanning effected by the probe, of thevibrating surface (connection 76). Level curves are thus obtained, whichcontrast on a graduated background, which is provided by the directpermanent actuation of the recorder by the detected signal (connection73).

A description is given above, in particular, for the use of the deviceof FIG. 1 for the analysis of a vibration. It may also be used forstatic measurements for example, the measurement of dimensions of partsin the process of being worked. Phaseshifting circuits 3-4-5-10 may thenbe disconnected. On the other hand, in this case, it will be necessaryto provide the device with various additional circuits, some of whichwill be described later. In particular, the device may comprise severalacoustical probes having well defined positions and means for switchingthem onto the transmitting-receiving and measurement circuit so as tocompare various dimensions with one another.

When it is desired to take precise dimensional measurements with respectto a reference standard, the differential assembly shown in FIG. 8 isadvantageously used.

This assembly comprises two transducers 40 and 41 respectively connectedto two transmitting-receiving electronic devices 42-43.

The pulse transmitters comprising these devices are synchronized by acommon clock generator 44, while their receiver outputs are respectivelyconnected to two gates 45 and 46. These gates are controlled by aflip-flop 47, itself tripped by clock generator 44, and they actuate aflip-flop 48. The signals originating from flip-flop 48 are applied to arecording or measurement device 49, which is advantageously a counter ofthe digital type fed with recurrent pulses generated by a clock andtransmitted through a gate for the duration of the signals (j).

FIG. 9 shows the wave forms at different points of the assembly:

at (a), the signal originating from receiver 43 at (b), the signaloriginating from receiver 42 at (c), the signal originating fromflip-flop 47 at (d), the signal originating from gate 45 at (e), thesignal originating from gate 46 at (f), the signal originating fromflip-flop 48 The assembly shown in FIG. 5 functions as follows:

Acoustical probes 40 and 41 are supported so that their transmittingsurfaces are rigorously in the same plane, parallel to the support ofstandard S of part P.

The first echo R provided by probe 41 is separated from thecorresponding transmission pulse E, by a time interval T, d,/c, c beingthe speed of ultrasonic propagation in air, while the first echo Rprovided by probe 40 is separated from the corresponding transmissionpulse E by a time interval l" d lc.

Gates 45 and 46 transmit output signals from devices 43 and 42 toflip-flop 48 only outside the time intervals defined by the gatingpulse, having wave form (0), FIG. 9, which is applied to them byflip-flop 47. In this way, only echoes R, and R are transmitted, as canbe seen at (d) and (e), FIG. 9, exclusive of transmission pulses andinterfering signals which may follow the latter.

Flip-flop 48 is switched into state 1 by the leading edge of R,, andrestored to state by the leading edge of R so that it generates pulse(j) of width At T, T, 1/0 (d d,). Time At is measured by device 49,which, therefore, indicates the difference 11, d, between the respectivethicknesses of the part and standard.

The assembly at FIG. 8 may be used for the measurement of the flow rateof a given liquid. Indeed, if both probes are immersed into the liquid,their transmitting faces being placed face to face at a distance d, asshown in FIG. M), each one obviously receives the acoustical pulseemitted by the other, after this pulse has travelled a distance d,either in the direction of the current or counter-currently.

However, the speed of propagation of this pulse is, for one of theprobes, c v cos 0 and, for the other one, 0 v cos 0, 0 being the speedof ultrasonic propagation in the resting liquid, v the speed of theliquid, and the angle included between the probe axis and this speed.

As a result, the width of pulse (1) originating from flip-flop 48 is:

This quantity, provided v/c is sufficiently small, is approximatelyequal to v cos0 2d/c2, and therefore provides a measure of v cos6.

It should be noted that this method of acoustical measurement of a flowrate does not require a vary precise setting of the two probes. This isnot true for the known method which consists in using a first probecouple respectively transmitting and receiving, each one arrangedopposite one another in the direction of the current and a second probecouple, respectively receiving and transmitting, each one similarlyarranged opposite one another in the direction of the current. Thisknown method, in practice, becomes unusable as soon as a relatively highdegree of precision in measurement is required.

When a very small thickness is to be measured in particular, if the partunder study is moving rapidly, it is advantageous, instead of using aconventional acoustical probe, to use the assembly ofFlG. ii.

In this assembly, a probe couple, 50-51, is used to measure the sum ofdistances D, and D respectively between their transmitting surfaces,each one arranged opposite one another in a parallel direction to thefaces of part M, and said faces, while a third probe 52 measuresdistance D between its transmitting surface, distance D being equal tothat separating probes l 5ll.

Probe 50 is actuated by pulses (E,, FIG. 9) provided by atransmitting-receiving device 53. Echoes R, generated by reflection ofultrasonic pulses from the surface of part M are transmitted to anamplifier 54, and then, through a gate 55, to a transmitting-receivingdevice 56. Pulses E transmitted by the transmitter of device 56 are thussynchronized on R,. Indeed, gate 55 is temporarily blocked by a gatingpulse generated by a monostable unit 57, which is tripped by asynchronization generator 58, which also synchronizes the emission ofpulses E,. The duration of this gating pulse is such that gate 55transmits echoes R,; pulses E actuate probe 51 and echoes R generated byreflection of ultrasonic pulses from the surface of part M, aretransmitted to an amplifier 56a and then, through a gate 56b to ameasuring device 5%. Gate 561; temporarily blocked by a monostable unit56c tripped by echo R, originating from gate 55, and generating a gatingpulse of a width such that gate 56b transmits R and not E FIG. 12 shows,respectively at (A) and (B), the wave forms at the output of gates 55and 56b.

Generator 58 synchronizes the emission of pulses E through a thirdtransmitting-receiving device 60, which actuates probe 52. Echoes R,generated by reflection of ultrasonic pulses on reference surface R(waveform C.), FIG. 12) are transmitted to measuring device 59, throughan amplifier 6i and a gate 62. The latter is blocked for a sufficienttime to avoid transmitting E through a gating pulse generated by amono-vibrator 63 tripped by E, and, to this effect, connected to device60.

Generator 58 comprises a circuit connected to a zero resetting input ofdevice 59.

Examination of FIG. 12 shows that time interval E, R is equal to k (D,D,), k being a constant, while time interval E R is equal to kD. Device59 is arranged so as to indicate time interval R R which is proportionalto D (D, D i.e., to the thickness e of the part.

The device of FIG. ll may be used to measure the internal diameter of atube: in that case, both probes 50 and 51 will be coupled so as to becentered on a diameter of the tube, and probe 52 will not be put intooperation. Device 59 will then measure time interval E,R which isproportional to the sum D, D of the distances of the free transmittingfaces of the two probes to the two extremities of said diameter. Inorder to obtain the diameter of the tube, the total thickness of bothcoupled probes will have to be added to this sum. It is selfevident thatvarious modifications may be introduced into the devices described andshown, without departing from the spirit of the invention.

What I claim is:

E. An apparatus for determining distances in a medium through reflectionof ultrasonic pulses on a reflecting surface within said medium, saidapparatus comprising transmitting means for transmitting an ultrasonicpulsed beam at an accurately predetermined point on said surface andreceiver means for receiving echo pulses from said reflecting surfaces,said transmitting means including at least one transducer probeessentially consisting of a substantially spherical active layer made ofpiezoelectric material inserted between a block made of a substancehaving a substantial damping coefiicient with respect to ultrasonicpulses generated by said active layer and a further quarter-wavelengthultrasound transmissive layer having a spherical concave surface, saidconcave surface forming a transmitting surface of the transducer andhaving a curvature adapted for focusing the said ultrasonic beam at afocal point located at a distance of a few millimeters from the saidtransmitting surface, said receiver means being connected to saidtransducer probe, said apparatus further comprising first and secondunits secured in symmetrical positions with respect to the transducerprobe, each of said units comprising a light source and an opticalsystem cooperating with said light sources for directing towards thesaid focal point, a light beam converging at the said focal point,whereby the two light beams and the ultrasonic pulsed beam converge atthe said focal point.

2. An apparatus according to claim 1, more particularly adapted foranalyzing a vibration of said reflecting surface, said apparatus furthercomprising pick-up means for detecting the said vibration, said pick-upmeans generating a periodic electrical signal, phase-shifting means forapplying a progressing phase-shift to said electrical signal, meansconnecting said phase-shifting means to said transmitting means, forcontrolling the transmission of said ultrasonic pulsed beam from thephase-shifted electrical signal. and means, connecting said pick-upmeans to said phase-shifting means for doubling the frequency of saidperiodic electrical signal and for adjusting said phaseshift so as toobtain the coincidence of the pulses of said ultrasonic pulsed beam withthe peak amplitude of the said vibration.

3. An apparatus according to claim l, more particularly adapted formeasuring a small thickness, wherein said transmitting means comprisefirst second and third transducers, a reference reflector, being locatedin the plane of the emitting surface of the first transducer, the secondand third transducers having their emitting surfaces located in afurther plane at a predetermined distance from said reference reflector,the emitting surface of the second transducer being located oppositethat of the first transducer, did transmitting means further comprisingfirst, second and third pulse transmitting devices respectivelyconnected at said first, second and third l 0 mission by the firsttransmitting device with the pulse transmission by the thirdtransmitting device and means for measuring the time interval separatingthe reception of echoes by the second and third echo receivers.

1. An apparatus for determining distances in a medium through reflectionof ultrasonic pulses on a reflecting surface within said medium, saidapparatus comprising transmitting means for transmitting an ultrasonicpulsed beam at an accurately predetermined point on said surface andreceiver means for receiving echo pulses from said reflecting surfaces,said transmitting means including at least one transducer probeessentially consisting of a substantially spherical active layer made ofpiezo-electric material inserted between a block made of a substancehaving a substantial damping coefficient with respect to ultrasonicpulses generated by said active layer and a further quarter-wavelengthultrasound transmissive layer having a spherical concave surface, saidconcave surface forming a transmitting surface of the transducer andhaving a curvature adapted for focusing the said ultrasonic beam at afocal point located at a distance of a few millimeters from the saidtransmitting surface, said receiver means being connected to saidtransducer probe, said apparatus further comprising first and secondunits secured in symmetrical positions with respect to the transducerprobe, each of said units comprising a light source and an opticalsystem cooperating with said light sources for directing towards thesaid focal point, a light beam converging at the said focal point,whereby the two light beams and the ultrasonic pulsed beam converge atthe said focal point.
 2. An apparatus according to claim 1, moreparticularly adapted for analyzing a vibration of said reflectingsurface, said apparatus further comprising pick-up means for detectingthe said vibration, said pick-up means generating a periodic electricalsignal, phase-shifting means for applying a progressing phase-shift tosaid electrical signal, means connecting said phase-shifting means tosaid transmitting means, for controlling the transmission of saidultrasonic pulsed beam from the phase-shifted electrical signal. andmeans, connecting said pick-up means to said phase-shifting means fordoubling the frequency of said periodic electrical signal and foradjusting said phase-shift so as to obtain the coincidence of the pulsesof said ultrasonic pulsed beam with the peak amplitude of the saidvibration.
 3. An apparatus according to claim 1, more particularlyadapted for measuring a small thickness, wherein said transmitting meanscomprise first second and third transducers, a reference reflector,being located in the plane of the emitting surface of the firsttransducer, the second and third transducers having their emittingsurfaces located in a further plane at a predetermined distance fromsaid reference reflector, the emitting surface of the second transducerbeing located opposite that of the first transducer, did transmittingmeans further comprising first, second and third pulse transmittingdevices respectively connected at said first, second and thirdtransducers and said receiver means comprising first, second and thirdecho receivers respectively connected to the first, second third thirdtransducers, the apparatus further comprising means for synchronizingthe pulse transmission from the secOnd transmitting device with theechoes received by the first echo receiver, means for synchronizing thepulse transmission by the first transmitting device with the pulsetransmission by the third transmitting device and means for measuringthe time interval separating the reception of echoes by the second andthird echo receivers.